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74HCT193 - Is it Fried?

Discussion in 'General Electronics Discussion' started by Raven Luni, Aug 20, 2012.

  1. Raven Luni

    Raven Luni

    798
    7
    Oct 15, 2011
    Greetings,

    This is related to my current bat detector project but it seems to merit a topic of its own since its a discussion about a particular component on its own.

    Anyway, after noticing a weak output from the Q2 pin (the 4 bit), I took some measurements. The state was changing between about 4.2V and 4.8V. The other outputs were fine (changing between approx 5V and a few millivolts). I also noticed a resistance of about 7.2k bwtween Q2 and the count up pin. (all other resistances between outputs and this pin read either infinite or several Megohms.)

    All outputs are completely isolated and not connected to anything else. So - is it fried, or is there maybe something quirky about the internals of this IC anyone here would know about?

    Being in a hurry I soldered it directly to the board instead of using a socket, so if it comes to the worst I suppose I could compensate by offsetting and amplifying.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,174
    2,690
    Jan 21, 2010
    This circuit?

    [​IMG]

    I can't see how the chip could be damaged. A good trick is to place a low value resistor (say 10R) in series with the Vdd lead. You can use this to monitor current to the chip. Many cases of failure will lead to high(er) quiescent current consumption.

    Also I'll note that the diagram says 74HC193 and you say you're using a 74HCT193 -- a quite different beast.

    HCT has TTL compliant inputs, these change state at unusually low (for CMOS) values.
     
  3. Raven Luni

    Raven Luni

    798
    7
    Oct 15, 2011
    Thats mostly for illustrative purposes and I used livewire to draw it (piece of crap that it is but at least its good for something) and its component selection is limited and uneditable. Its definitely an HCT I'm using. The Q2 and count up pins are right next to each other and that low resistance (unpowered) between them is suspicious (and since none of the other Qs have this it suggests something might have melted. There are a number of ways this could have happened: the fact that it was soldered to the board without a socket and the various shorts etc. that had to be cleaned up due to messy soldering or tracks not cut in the right places.
     
  4. Raven Luni

    Raven Luni

    798
    7
    Oct 15, 2011
    Yep - it was cowed. I took measurements from another one and they were all what I would expect. So I took out the offending article and put a socket in its place with the fresh 193 and it all works perfectly now.
     
  5. Raven Luni

    Raven Luni

    798
    7
    Oct 15, 2011
    Interestingly enough I was putting everything together in the case and noticed the same quiet output from the same pin. Replacing it again (this time with an HC rather than an HCT) fixed the problem. There is definitely some kind of vulnerability around this pin - most likely thermal (I had been soldering in that area again to fix a few things).
     
  6. gorgon

    gorgon

    603
    23
    Jun 6, 2011
    Or it could be static electricity from the handling, or soldering iron? Is the soldering iron grounded in any way?

    TOK ;)
     
  7. Raven Luni

    Raven Luni

    798
    7
    Oct 15, 2011
    I suspect the soldering iron has an earth connection given that its a mains appliance with exposed metal bits. Static seems unlikely. I've handled loads of these before, even got 1 stuck to the bottom of my shoe once and had to bend all the pins back into shape - still worked :)
     
  8. gorgon

    gorgon

    603
    23
    Jun 6, 2011
    I just had a second look at your schematics. Could it be that the speaker induces negative voltage spikes and pull the output below 0V when turned off? How about putting a diode over the speaker to remove this possibility?

    Is there a reason that you have put the speaker in the emitter-gnd and not collector-+V circuit? You will have a higher output if you use the inverting mode, in the collector - +V side. But the diode protection is valid for both.

    TOK ;)
     
  9. Raven Luni

    Raven Luni

    798
    7
    Oct 15, 2011
    Hmm - I always thought the general rule for transistors was emitter side for current, collector side for voltage. The output levels are satisfactory for the moment and adding a diode is easy enough. Thanks for the advice. Could you elaborate a bit on the transistor?

    And its also worth noting that I've since changed the base resistor to 1K and put a 33 Ohm on the collector
     
    Last edited: Sep 9, 2012
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